1. Field of the Invention
The present invention relates to a magnetic toner for use in an image forming method for visualizing an electrostatic latent image in electrophotography.
2. Description of the Related Art
On-demand printing has recently been increasingly required for an image forming apparatus such as a copier and a printer. A highly reliable toner realizing higher-speed printing and higher-quality images has thus been desired. Additionally, since environment of the use has diversified, a toner that can provide stable images, even used under various environments, has been desired.
Of the image-forming systems, a single-component development system is favorably used because the development system is a developing apparatus simply constructed and having fewer problems and has a long product-life and ease of maintenance.
Several techniques are known for the single-component development system. One of the techniques is jumping development using a magnetic toner. In the jumping development, a magnetic toner is frictionally charged, allowed to jump on a photoreceptor by application of an alternate-current developing bias to develop an electrostatic latent image formed on the photoreceptor into a visible toner image.
To obtain a high-quality image by the jumping development, electrification characteristics of a magnetic toner are critical. However, in general, the electrification characteristics of a magnetic toner greatly vary depending upon the operating environment. For example, under a normal-temperature and low-humidity environment, the electrification amount of a magnetic toner is likely to increase. The magnetic toner having a high electrification amount easily attaches onto a sleeve by image force, causing so-called sleeve contamination. The magnetic toner attached onto the sleeve interrupts frictional electrification between another magnetic toner and the sleeve. As a result, the density of toner decreases and a high-quality image may not be obtained.
Furthermore, under a high-temperature and high-humidity environment, the electrification amount of a magnetic toner is likely to decrease. In addition, a developing apparatus is often left alone for about a week without being operated, taking a long vacation of an office, etc., into consideration. If a developing apparatus is left alone for a week, electrification of a magnetic toner is relaxed and electrostatic repulsion decreases. Agglomeration of toner particles easily occurs. As a result, the toner agglomerate flies to a non-image forming area. This is called large-particle fogging, which often has a harmful effect upon an image.
Furthermore, large-particle fogging is likely to occur, if electrification is relaxed after leaving a toner alone, even under a normal-temperature and low-humidity environment, whereas the density of toner decreases in some cases, even if a sleeve is slightly contaminated, under a high-temperature and high-humidity environment.
To deal with such problems, various studies have been conducted in order to control electrification characteristics of a magnetic toner. Particularly, in a magnetic toner used in a process for developing an image following application of an alternate-current developing bias like a jumping development, values of a dielectric constant, a dielectric loss factor and dielectric loss tangent are important. Several techniques for obtaining a stable image by controlling these characteristic values have been proposed.
For example, Japanese Patent Application Laid-Open No. 2006-030881 discloses a magnetic toner having excellent developability, even under a high-temperature and high-humidity environment, which is obtained by controlling a dielectric loss tangent and an average circularity to fall within a desired range. However, in view of the dielectric loss tangent value proposed in this patent document, an electrification amount is likely to increase and sleeve contamination may occur under a normal-temperature and low-humidity environment.
Furthermore, Japanese Patent No. 4136899 discloses a magnetic toner having excellent dot reproducibility and less fogging, which is obtained by defining the dielectric constants of the magnetic toner and an external additive together with the particle diameter, true specific gravity, etc., of a magnetic toner. However, the magnetic toner proposed in this patent document exhibits satisfactory developability in a normal operation time but if the magnetic toner is left under a high-temperature and high-humidity environment for a week, large-particle fogging often generates. Further improvement is required.
Furthermore, Japanese Patent No. 4307297 discloses a magnetic toner using a resin having a glycidyl group and providing a stable image even in a two-sided output operation by defining a dielectric constant and a dielectric loss tangent. However, also in the magnetic toner proposed in this patent document, if the magnetic toner is left alone under a high-temperature and high-humidity environment for a week, large-particle fogging may occur.
As described above, there are a great many technical problems for obtaining a stable image under environments different in electrification characteristics of a magnetic toner, such as under a normal-temperature and low-humidity environment and under a high-temperature and high-humidity environment. In addition, if a toner is left alone for a week, although it is not a rare case in daily operation, since electrification relaxation of a magnetic toner proceeds, conditions of the magnetic toner become particularly severe. To obtain a stable image under these environments, a further improvement is required.
An object of the present invention is to provide a magnetic toner with which the aforementioned problems have been overcome. More specifically, an object of the present invention is to provide a magnetic toner providing a stable image while suppressing sleeve contamination and large-particle fogging even under environments different in electrification characteristics of a magnetic toner.